Apple likes to verify software updates, which is not so good for your jailbroken iDevice. Cydia generally circumvents the problem, but their servers aren't exactly working at the moment. To avoid problems, you need to back up something known as your SHSH file. Here's how to do it.

Your ability to jailbreak your iDevice is in jeopardy because Cydia's SHSH servers are currently unavailable. What does this mean? I'll let BigBoss explain what an SHSH file is for:

For those of you unaware of the Apple firmware update policy, you can only restore firmware version that Apple approves. When you restore in iTunes, it goes to Apple's web site and verifies if you are eligible for this specific version. This was solved by Cydia storing your SHSH (the "is it ok to restore this version" request response from Apple). This has been ok for the last year.

With Cydia's servers unavailable, this process can't happen. In theory they'll be back up and you'll be fine, but to avoid losing your jailbreak you're going to want to back up your SHSH file(s) with a little application called TinyUmbrella (download it for Windows and Mac OS X). If you need to restore and re-jailbreak your iDevice, you can then use TinyUmbrella as your verification server instead of relying on Cydia. The video above demonstrates the process, but here's a basic outline of the steps:

Launch TinyUmbrella with your iDevice connected. Turn off "Web Sharing" in Mac OS X if you have it on and intend to use TinyUmbrella's servers.

Click the button to save your SHSH files. This will take some time.

Go to the advanced tab and uncheck "Request SHSH from Cydia" and "Set Hosts to Cydia on Exit," then click "Apply Settings."

That's all you need to do to back up your SHSH files. Despite what that video says, you do not need to enter recovery mode to start the process. That was a mistake, sorry!

UPDATE: And, of course, Cydia's now back up and running. Nonetheless, it's not a bad thing to back up your SHSH anyway so you have them in case of a future outage.